They have logged a call with Telkom, no joy, I have tested with three different routers, no joy, 2 different ISP's joy! Please peruse the rest of this subforum, you will see that I am not alone. RPM and others have experienced the same line speed volatility.
I don't understand the SNR and db values that one can get from one's router BUT what I do understand that if read correctly then they can say whether it is a bad physical line or what. So, if you posted the stats when bad and when good I am sure someone will be able to give you an answer.